1. Field of the Invention
The present invention relates to a charged particle beam instrument, such as an electron probe microanalyzer or a scanning electron microscope.
2. Description of the Related Art
In a charged particle beam instrument, such as an electron probe microanalyzer or a scanning electron microscope, a charged particle beam emitted from a charged particle beam source is accelerated and focused onto a specimen by a condenser lens system and an objective lens. As the charged particle beam hits the specimen, X-rays and secondary particles are produced, and these are detected.
In this kind of instrument, the current of the charged particle probe made to hit the specimen is stabilized. FIG. 1 is a diagram schematically illustrating this probe current-stabilizing function. A charged particle beam CB produced by a charged particle beam source (not shown) and accelerated is sharply focused onto a specimen 3 by a condenser lens system 1 and an objective lens 2.
A detection aperture 4 is located between the condenser lens system 1 and the objective lens 2 and detects an outer portion of the charged particle beam. The output signal from the detection aperture 4 is amplified by a feedback device 5 and supplied to a control portion 6 for the condenser lens system 1 for adjusting the probe current.
The control portion 6 adjusts the strength of the condenser lens system 1 according to the magnitude of a reference signal and the magnitude of the output signal from the feedback device 5. A feedback loop is formed in this way. Therefore, the current of the charged particle beam probe P impinging on the specimen 3 can be kept constant in principle if the current density of the charged particle beam does not vary.
To establish negative feedback (i.e., to prevent positive feedback as described in Japanese Patent Laid-Open No. 183044/1989), an aperture for limiting peripheral portions of a charged particle beam exiting from the condenser lens system is placed ahead of the detection aperture 4 as described in Japanese Technical Review 82-7798. This aperture is omitted in FIG. 1.
The detection aperture 4 can also be designed to act also as an objective aperture for controlling the probe current and the divergence angle of the probe.
As mentioned previously, where negative feedback is applied to the condenser lens system 1, if the exciting current supplied to the condenser lens system 1 is varied so as not to vary the probe current, the position of the focal point of the condenser lens system 1 automatically changes from the state indicated by the solid line to the state indicated by the broken line. It is now assumed that some change occurs in the charged particle beam source and that the probe current should vary from Ip by ΔIp. However, the negative feedback varies the distance between the detection aperture 4 and the focal point, thus maintaining the probe current Ip constant.
In spite of this, an adjustment of the condenser lens system 1 moves the focal position of the charged particle probe P on the specimen out of the specimen surface by Δb. The spread portion Δd1p of the probe diameter due to the feedback adds to the final probe diameter dp.
It is assumed that the objective lens 2 has an object distance of a and an image distance of b. If the focal distance fOL of the objective lens 2 is constant, the following relation holds:db/da=−M2where M (=b/a) is the magnification of the objective lens. Therefore, when the object distance varies by a small distance of Δa, the image distance deviates by Δb, which is given by:Δb=−M2·Δa
That is, the image distance deviation Δb can be reduced by combining the lenses so as to reduce the magnification M (=b/a). It can be seen, however, that the deviation Δb cannot be reduced to any desired small value, because the number of lenses is finite, and because the microscope column has a finite length.
On the other hand, in an instrument equipped with a charged particle beam source of low brightness, the final probe diameter dp is not thin. Therefore, the spread Δd1p of the probe diameter due to negative feedback presents no serious problems. In contrast, emission of a charged particle beam from a charged particle beam source of high brightness (e.g., field emission, electron emission, such as Schottky emission, and ion emission due to field ionization or electrolytic dissociation) can produce a quite thin final probe diameter dp. Consequently, the spread Δd1p of the probe diameter due to negative feedback can no longer be neglected.
Furthermore, in a charged particle beam source of high brightness, the emission current tends to vary. This increases the amount of correction made by negative feedback. This, in turn, increases the spread Δd1p of the probe diameter, thus increasing the amount of defocus.
A charged particle beam source of high brightness is adopted to obtain a small probe diameter. This object cannot be achieved due to the spread Δd1p of the probe diameter, which, in turn, is caused by negative feedback that is used to obtain a stable probe current.